The effect of cryogenic temperatures on the mechanical properties and fracture mechanism of SnAgCu-SnPb mixed solder joints was investigated. The results showed that the tensile strength of mixed solder joints first increased with the increase of Pb content and reached its maximum at 22.46 wt.% Pb; subsequently, it decreased as Pb content increased. However, cryogenic temperatures improved the tensile strength of the solder joints. Both Pb content and cryogenic temperature caused the fracture mode of the mixed solder joints to change; however, temperature remained the main influencing factor. As the temperature fell from 298 to 123 K, the failure pattern in the solder joints transformed from ductile fracture to quasi-ductile fracture to quasi-brittle fracture and finally, to brittle fracture.